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Conference Papers Discrete Mathematics and Theoretical Computer Science Year : 2013

## Rational Catalan Combinatorics: The Associahedron

Drew Armstrong
• Function : Author
• Function : Author
Nathan Williams

#### Abstract

Each positive rational number $x>0$ can be written $\textbf{uniquely}$ as $x=a/(b-a)$ for coprime positive integers 0<$a$<$b$. We will identify $x$ with the pair $(a,b)$. In this extended abstract we use $\textit{rational Dyck paths}$ to define for each positive rational $x>0$ a simplicial complex $\mathsf{Ass} (x)=\mathsf{Ass} (a,b)$ called the $\textit{rational associahedron}$. It is a pure simplicial complex of dimension $a-2$, and its maximal faces are counted by the $\textit{rational Catalan number}$ $\mathsf{Cat} (x)=\mathsf{Cat}(a,b):=\frac{(a+b-1)! }{ a! b!}.$ The cases $(a,b)=(n,n+1)$ and $(a,b)=(n,kn+1)$ recover the classical associahedron and its Fuss-Catalan generalization studied by Athanasiadis-Tzanaki and Fomin-Reading. We prove that $\mathsf{Ass} (a,b)$ is shellable and give nice product formulas for its $h$-vector (the $\textit{rational Narayana numbers}$) and $f$-vector (the $\textit{rational Kirkman numbers}$). We define $\mathsf{Ass} (a,b)$ .

#### Domains

Discrete Mathematics [cs.DM]

### Dates and versions

hal-01229692 , version 1 (17-11-2015)

### Identifiers

• HAL Id : hal-01229692 , version 1
• DOI :

### Cite

Drew Armstrong, Brendon Rhoades, Nathan Williams. Rational Catalan Combinatorics: The Associahedron. 25th International Conference on Formal Power Series and Algebraic Combinatorics (FPSAC 2013), 2013, Paris, France. pp.933-944, ⟨10.46298/dmtcs.2355⟩. ⟨hal-01229692⟩

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